Interesting thought process here. Once you see this mate-in-3, the mate in 2 solution (which uses much the same mating situation - Queen, protected by the d1 Rook, mates on an adjacent-diagonal square) becomes all the more obvious, But once we see a sure-fire forced mating combo, it is hard to shift and look for something better. Only if we are told there is a "mate-in-2" solution do we look any burther,

Thanks dufferps. You're absolutely right of course. My stubbornness about my 3 move mate WAS difficult to let go of---and did add time to my search for the simpler solution. Good call.